‘Almost certainly you wouldn’t pop into a dinosaur-ruled 2017 and find a Tyrannosaurus rex staring down at you.’
Photograph: J Scott Applewhite/AP

Sixty-six million years ago, dinosaurs had an exceptionally bad day. A chunk of space rock nine kilometres across smacked into what’s now Mexico’s Yucatán peninsula, instantaneously triggering an extinction event that for ever changed the nature of life on Earth. This is one of the rare moments when we can look back at a pivotal point, where history veered off on an unexpected path. Maybe that’s why we’ve been so obsessed with what would have happened if Tyrannosaurus, Triceratops and other Cretaceous celebrities hadn’t gone extinct.

The urge to hit rewind on deep time and wonder about the fate of the non-avian dinosaurs is even stronger now that a study has determined that the terrible lizards were not only unlucky, but extraordinarily unlucky. The asteroid hit rock layers so rich in hydrocarbons that the impact threw massive amounts of soot and sulphate aerosols into the atmosphere, putting the chill on organisms that survived the initial blast. And the kicker? Rocks with such amounts of hydrocarbons cover only about 13% of the Earth’s surface.

The asteroid didn’t even have to miss the Earth to give non-avian dinosaurs a stay of execution. The immense chunk of stone could have struck almost anywhere else and not triggered the fifth mass extinction in life’s history. And that leads us, as a species obsessed with our own fate in the face of an uncertain future, to ask: what if non-avian dinosaurs had survived?

‘The Cretaceous world was not inhabited by dinosaurs alone. It’s easy to forget about the flying pterosaurs.’ Photograph: Luis Rey/BA/PA

Scientists, artists and film-makers have been wondering about this evolutionary twist ever since the end-Cretaceous mass extinction was recognised as a reality in the late 20th century. The Scottish author Dougal Dixon created an entire menagerie of speculative reptiles, The New Dinosaurs, around this idea, and the 2015 Disney/Pixar movie The Good Dinosaur is based on more or less the same conceit. But given another 66m years, dinosaurian history probably wouldn’t have turned out the same way as in the movies – and nor would we be around to watch these new species roam the planet.

In trying to find our way through this lost timeline, it’s important to remember that the Cretaceous world was not inhabited by dinosaurs alone. It’s easy to forget about the flying pterosaurs, strange marine reptiles, and, naturally, fuzzy mammals scurrying through the undergrowth. The end-Cretaceous catastrophe wasn’t just a bad day for dinosaurs but almost all forms of life: there were mass extinctions of birds, lizards and snakes, mammals, coil-shelled ammonites and reef-building clams called rudists. Entire ecological webs were torn down and were left to reform from the tatters.

It was this environmental wound that allowed mammals to thrive in new ways, even setting the stage for our own ancestors. But if dinosaurs had continued to hold sway in the terrestrial realm, we never would have evolved. Our early primate forebears would have been shunted along different evolutionary routes we can only guess at.

There’s no reason to think that dinosaurs would have vanished and ceded the world to mammals if the extinction had been cancelled. There were over 80m years between the time of Brontosaurus and Tyrannosaurus, but it’s only been 66m years since the last of the non-avian dinosaurs disappeared. We could fit the entire age of mammals between those two famous dinosaurs with room to spare. (And I hasten to point out that dinosaurs are still with us in the form of birds, extending their family’s 235m-year tenure.)

Non-avian dinosaurs had survived sweeping changes to climate, shifting continents, and the ticking turnover of species as evolution and extinction work simultaneously. Of course they would have survived to what we know as the present day.

'There’s no reason to think that dinosaurs would have ceded the world to mammals if the extinction had been cancelled.'

What those dinosaurs would look like, however, is another question. There’s no way to know, especially as evolution only generates what works in the moment and has no sense of future planning. Looking at the last days of the Cretaceous, it’s difficult to say if the birds, tyrannosaurs, horned dinosaurs, Ankylosaurs, duck-billed dinosaurs and sauropods would have given rise to entirely new forms of dinosaurian life. But almost certainly you wouldn’t pop into a dinosaur-ruled 2017 and find a T rex staring down at you.

And here’s where science fiction usually comes in. What if some form of dinosaur evolved to be like us? Maybe it’s a kind of saurian wish-fulfilment on our part – so many of us imagine playing as dinosaurs as children – but humanoid dinosaurs are a recurring motif in these alternative histories, often looking like the Sleestaks from the old Land of the Lost TV programme. The palaeontologist Dale Russell and the taxidermist Ron Seguin imagined the raptor-like dinosaur Stenonychosaurus evolving into a big-brained, bipedal “Dinosauroid”, a visual argument that increasingly smart dinosaurs would follow an evolutionary path similar to our own.

We don’t really need to replay the geologic tape to see what smart dinosaurs would look like, though. They’re already around us. Ravens, magpies, African grey parrots and other avian dinosaurs are exceptionally intelligent but they hop, flutter, and squawk in bodies very different to our own. A supersmart dinosaur wouldn’t resemble anything humanoid. If dinosaurs with a mental toolkit similar to ours ever evolved, they’d probably look little different to a crow.

And that underscores something every dinosaur fan can be thankful for. Mass extinction or not, we’re still very much in the age of dinosaurs. Think of that the next time you spot a jackdaw hopping around the garden.